As is known, in order to produce tubular films the molten plastic material is extruded through a die comprising a central core and an outer ring surrounding said core at a close distance, so as to define an annular slit through which the tubular film is extruded.
The molten plastic material is directed toward said slit through a number of channels provided at the interface of said core and said outer ring or sleeve, channels which are uniformly distributed over its entire perimeter and which are branching out when approaching the extruding slit.
Despite the precision these dies are built with, it frequently happens that the thickness of the tubular film is not perfectly uniform, for instance as a result of an irregular flow of the material.
In order to limit this drawback, some dies have been developed in which the outer panel is built with a number of separate sectors each of which is associated with a heating element, and certain other devices mechanically acting on the ring so as to locally deform it according to the shifts caused by the motions generated by the technical deformations of certain sectors of the ring.
An example of such structures is shown in a simplified manner in FIG. 1, which illustrates, in a cross-sectional view, a known device for the localized adjustment of the extrusion slit.
The number 1 in FIG. 1 indicates the core of the die, number 2 the body of the outer ring surrounding the core and number 3 the annular extrusion slit expelling the material.
A multiple number of inserts 4 are radially arranged around the ring and mounted on an annular supporting structure indicted by the number 5.
An adjusting screw 6 allows regulating the position of the inserts, by approaching or moving them away from the ring 2.
Once this setting position has been defined, the inserts are tightened down in their positions.
The inside of each insert holds an electrical resistance 23 and a conduit 7 allowing the passage of cooling fluid.
A certain space is allowed between the ring 2 and the insert 4, so as to allow the insert to freely expand when heated.
A tie-down element such as a small anchor 8 or similar rigidly connects each insert of the ring 2.
This system allows it to locally vary the thickness of the extrusion slit, while heating or cooling the insert 4. It has the drawback, however, of combining two conflicting features.
In order to locally reduce the thickness of the extruded tubular material, the insert must in fact be heated so as to allow it to expand and push in the lip of the ring in the direction of the central core 1, thus slightly restricting the outlet slit 3.
A change in the slit diameter of about two tenths of a millimeter requires an insert temperature rise of about 100-150.degree..
It may then happen that despite a provision for an appropriate heat insulation between the inserts and the ring, part of the evolving heat will pass through the ring and heat the outflowing material, making it more fluid and free-flowing and consequently boosting its flow rate.
The efficacy of the mechanical deformation imparted to the ring is consequently partly annulled by the viscosity reduction of the material, resulting from the temperature increase.